What does the La'av of "Lo Sisa Sheima Shav" entail?
Rashi #1 (citing Targum Onkelos) and Targum Yonasan: It is a prohibition against accepting Lashon ha'Ra (or speaking it Makos 23a). 1
Rashi #2: It is a La'av prohibiting a Dayan from hearing the words of one litigant before his opponent has arrived (and a litigant presenting his claim 2 to a judge before his opponent arrives - Sanhedrin, 7b and Mechilta). 3
Rashbam: It is a La'av prohibiting a. Dayan from accepting false testimony - by not thoroughly cross-examining the witnesses 4 and b. a witness from testifying together with false witnesses - even if there are two of them whose testimony will be accepted even without them.
R. Chaim Paltiel: It is a prohibition against the Dayanim accepting a fraudulant claim.
R. Bachyei: It is a prohibition against the Dayanim accepting a Shevu'ah from someone whom they suspect will violate it.
Pesachim, 118a: It is prohibition against speaking Lashon ha'Ra, accepting Lashon ha'Ra and testifying falsely against one's fellow-Jew. 5
Oznayim la'Torah: Although Onkelos tends to not differentiate between "Shav" and "Sheker", the word "Shav" generally refers to something that is obviously false - in which case the Torah here is forbidding even to listen to Motzi-Shem-Ra which he knows to be untrue and to remain silent, as that only encourages the speaker to continue telling tales about other people.
Because we read it as if it had written "Lo Sasi" - See Torah Temimah, note 2.
See Re'ei Devarim 13:9 (Rashbam).
Pesachim, Ibid.: And the Torah juxtaposes it to "la'Kelev Tashlichun oso" to teach us that anyone who speaks Lashon ha'Ra, accepts Lashon ha'Ra or who testifies falsely against his fellow-Jew deserves to be thrown to the dogs.
What does the La'av of "Al Tasheis Yadcha Im Rasha " entail?
Rashi (according to Divrei David 1 ): 'Do not promise to testify for someone who claims falsely (even if in the end, you do not testify)'.
Seforno: It is a prohibition against combining with a Rasha to sign on a document.
Yevamos 25a: It is a prohibition against testifying together with a Rasha.
Sanhedrin 27a: It is a prohibition against the Dayanim accepting testimony of a Rasha.
Mechilta #1: It is a prohibition against Reuven saying to Shimon 'Levi owes me two hundred Dinrim; and I have one witness. Come and combine with him and you and I will take one Manah each'.
Mechilta #2: Based on this Pasuk the refined people of Yerushalayim would not participate in a Seudah until they knew who would join them there and would not sign on a Get until they knew who would co-sign with them. 2
Ibn Ezra: It is a prohibition agains a witness asking someone else to testify falsely together with him.
R. Chaim Paltiel: It is a prohibition against the Dayanim accepting false testimony, and placing the blame on the head of the witnesses.
Bava Kama, 72b: It disquaifies the testimony of an Eid Zomem retroactively (from the time that he testifies) - because from the time that he testifies he is a Rasha, and the Torah writes "Al Tashes Yadcha im Rasha" ('Al Tashes Rasha Eid'.
Sanhedrin, 27a #1: It is a prohibition against a Mumar (an apostate) who eats Neveilos, whether it is for pleasure or out of spite - testifying, since he is a Rasha, and the Torah writes "Al Tashes Yadcha im Rasha" ('Al Tashes Rasha Eid'). 3
Sanhedrin, 27a #2: "Al Tasheis Yadcha ... " ('Al Tashes Chamas Eid'): It is a prohibiton against robbers, people who lend on interest and people who renegue on their Shevu'os testifying. 4
What is "Lih'yos Eid Chamas" coming to add?
Seforno: In the event that you testify together with a Rasha, it transpires that you are a single [Kosher] witness, and Beis-Din will pervert justice by exacting money from the litigant against whom you are testifying on the basis of your testimony alone.
Mechilta: An example of an Eid Chamas is someone who is asked by his Rebbe to join a second witness to extract a Manah from his debtor - even though one know that, for all the money in the world, his Rebbe would not lie.
Why is this Pasuk written after the Pasuk of T'reifah?
Hadar Zekenim: Because one who testifies falsely deserves to be thrown to the dogs. 1
Like Izevel, who commanded witnesses to testify falsely about Navos (Melachim 1 21:8-10), and who was subsequently eaten by dogs (Melachim 2, 9:36). Refer also to 23:1:1:6*.
